Different approaches are being applied to study the mechanism of action of thiamin pyrophosphate requiring enzymes and model systems for the mechanism of action of the coenzyme. The approaches include heavy atom kinetic isotope effect determinations on the decarboxylation of pyruvate by yeast pyruvate decarboxylase and by E. coli pyruvate dehydrogenase multienzyme complex, determination of the function of the multiple binding modes of Mg(II) and thiamin diphosphate to yeast pyruvate decarboxylase, and models for a catalytic function of the aminopyrimidine ring of the coenzyme. From these studies it is hoped that elucidation of the rate-limiting step of the enzymic decarboxylation, of the effect of metabolic regulators on multienzyme complexes and a better understanding of coenzyme function will result.